This invention relates generally to high pressure pumping systems, and more specifically to a damper for use with a solvent supply system utilized in high performance column liquid chromatography.
Chromatography is a separation method wherein a mixture of components (called the "sample" or "sample mixture") is placed at one end of a system containing a stationary phase and a mobile phase. Each component of the sample distributes itself as a separate zone in dynamic equilibrium between the two phases in a ratio characteristic of that component. As a result, the flowing mobile phase causes each individual component zone to migrate at a characteristic rate, and the zones become separated after a period of time.
There are various types of chromatography, e.g., liquid chromatography, gas chromotography, thin-layer chromatography, etc. The major differences between these various chromatographic methods are the physical state of the mobile phase (gas or liquid), and the manner in which the stationary phase is supported, e.g., whether coated on an inert granular material packed in a tube, coated on an inner wall surface, etc. In each method, the separation mechanism is essentially the same, i.e., distribution of the sample components between a mobile phase and a stationery phase. When the method is used for chemical analysis, a detector is commonly placed at the far end of the system, so as to monitor the passage of the component zones as they emerge from the system. The signal from the detector is displayed on a recording device such as a strip chart recorder, and the record indicates with qualitative and quantitative information regarding the components of the sample.
It is often desirable for a chromatographic system to provide high resolution (i.e., a large degree of component separation with narrow zones), evenly spaced component zones, rapid separation, and a satisfactory record from a very small sample. The behavior of the system described in these terms may be called the "performance" of the system. It is well known in the chromatography art to improve system performance by changing one of the following system variables during the course of the analysis: temperature, chemical composition of the mobile phase, and flow rate of the mobile phase. For example, in gas chromatography the temperature of the system is often varied as a preselected function of time. This technique is known as "temperature programming", and it improves the performance of the system, especially with samples containing components which boil over a wide temperature range. Analagous to temperature programming in gas chromatography is the use of "gradient elution" in liquid chomatography. Gradient elution refers to changing the chemical composition of the mobile phase (also called the "eluent" or "eluting solvent") as a function of time, thereby improving the performance of the system, especially with samples containing components which vary widely in chemical properties. The net effect of gradient elution is to shorten the retention time of compounds strongly retained on columns without sacrifice in separation of early eluting compounds. Further details regarding the fundamentals of gradient elution techniques may be found in numerous sources in the prior art, as, for example, in the publication by L. R. Snyder appearing in Chromatography Review 7,1 (1965).
A central concern pertinent to liquid chromatography apparatus of the type considered herein is one of providing a proper flow of solvent to and through the chromatography column. Thus in the past, numerous and varied approaches have been utilized for supplying solvents to high performance liquid chromatography columns. A key requirement in this connection is one of providing a relatively non-pulsating (i.e., a constant) flow of solvent -- in that the LC detector is sensitive to flow variations, and can provide erroneous readings and exhibit excessive noise in the presence of pulsing flow. Various approaches have been utilized in the past in order to enable such result; but in general, the prior art methodology directed at such end has involved highly expensive and overly complex mechanisms. Thus, in a typical example wherein a system is intended for operation in a gradient elution mode, i.e., by use of two distinct solvents, a dual pump arrangement may be utilized. Such arrangement requires two distinct pumps, including separate means for driving each of the pumps, which thus requires separate speed controls, etc.
In principle, it would seem that the cited problems arising in connection with the solvent pumping systems of the prior art might be overcome by use of a single cylinder arrangement in cooperation with a relatively small displacement volume reciprocating piston. A principal deterrent to the use of this arrangement, however, has been the fact that the ensuing flow will, by its nature, be pulsating -- particularly at low flow rates. Further, the nature of the pulses present in the flow is such that they are not easily removed by filtering, and the presence of such pulses can sharply limit the efficiency of the detector system. It should be understood in the foregoing connection that the word "piston" as used in this specification is intended to include both pistons where the seal remains fixed in relative position to the moving member and plungers where the seal is fixed with respect to the stationary cylinder.
In U.S. Pat. No. 3,985,021 to P. Achener et al. entitled HIGH PERFORMANCE LIQUID CHROMATOGRAPHY SYSTEM, which patent is assigned to the same assignee as the present application, there is disclosed a liquid chromatography system which is particularly useful in overcoming the aforementioned flow problems. The system includes a reservoir for a liquid mobile phase, a liquid chromatography column, reciprocating pumping means for pumping the mobile phase through the column, and motor means for driving the pumping means through successive reciprocation cycles. Means are provided further for controlling the rotational speed of the motor throughout the reciprocation cycle of the pump so as to provide preselected average rotational speeds over predetermined subintervals of each successive reciprocation cycle. Application of the control cycle is synchronized with the pumping cycle so that the speed control is properly applied over each successive reciprocation cycle.
In systems of the cited type, however, as well as in other high pressure liquid pumping systems incorporating reciprocating pumps, pulsation can to varying degrees still occur downstream of the pump thereby prompting interest in damping devices for further reducing or removing same. It has in the past been common to utilize for such purposes dampers which effectively constituted enlarged volumes, e.g. a hollow canister. Such prior art devices, however, introduced an undue amount of volume in the system -- which in LC systems interferred with purging and with generation of gradient changes.
In accordance with the foregoing, it may be regarded as an object of the present invention to provide a damper for use with a high pressure pumping system including a reciprocating pump, which damper is of simple, low cost construction, and yet is highly effective in damping pressure pulses.
An additional object of the present invention is to provide a canister type in-line damping device, which while serving very effectively to diminish or remove pulses that may remain following the outlet valve of the system pump nevertheless works with very limited volumes of the flowing liquid, thereby facilitating fast changes in solvent composition, and not impairing purging.